1. Field of the Invention
The invention relates to an implantable medical device in general and to an implantable heart stimulator in particular.
The implantable heart stimulator preferably is an implantable pacemaker or an implantable cardioverter/defibrillator (ICD), or a device for cardiac resynchronization (CRT-D).
2. Description of the Related Art
Heart stimulators such as cardiac pacemakers are medical devices, usually implantable, that can be connected to or that are permanently connected to electrode leads for delivery of electrical stimulations pulses to the tissue (myocardium) of a human heart. Dual chamber pacemakers are capable of generating stimulation pulses for the atrium and the ventricle of a human heart. Biventricular pacemakers usually are capable to stimulate at least three chambers of a human heart that is the right atrium, the right ventricle and the left ventricle.
In a dual chamber pacemaker, this is usually realized by placing electrodes in both the right atrium and right ventricle of the heart.
In a demand-type pacemaker these electrodes are coupled through intravenous and/or epicardial leads to sense amplifiers housed in an implanted pacemaker. Electrical activity occurring in these chambers can thus be sensed. When electrical activity is sensed, the pacemaker assumes that a depolarization following a contraction of the indicated chamber has occurred. If no electrical activity is sensed within a prescribed time interval, typically referred to as an atrial or ventricular escape interval, then a pulse generator, also housed within the pacemaker housing, generates a stimulation pulse that is delivered to the indicated chamber, usually via the same lead as is used for sensing.
Separate stimulation pulse generators are usually provided for each heart chamber (atrium or ventricle) to be stimulated.
A control unit triggers the generation of a respective atrial or ventricular stimulation pulse according to a pre-programmed, variable timing regime in order to provide for adequate timing of the stimulation pulses.
A stimulation pulse to the myocardium may cause a contraction of a respective heart chamber, if the myocardium of that chamber is not in a refractory state and if the stimulation pulse intensity is above the stimulation threshold of said myocardium. A sub-threshold stimulation pulse will not cause a cardiac contraction even if delivered to the myocardium in its non-refractory state.
Depending on the mode of operation, a pacemaker only delivers a stimulation pulse (pacing pulse) to a heart chamber (atrium or ventricle) if needed, that is, if no natural excitation of that chamber occurs. Such mode of operation is called an inhibited or demand mode of operation since the delivery of a stimulation pulse is inhibited if a natural excitation of the heart chamber is sensed within a predetermined time interval (usually called escape interval) so the heart chamber is only stimulated if demanded.
In a demand mode, the pacemaker monitors the heart chamber to be stimulated in order to determine if a cardiac excitation (heartbeat) has naturally occurred, such natural (non-stimulated) excitation, also referred to as “intrinsic” or “signs” cardiac activity, are manifested by the occurrence of recognizable electrical signals that accompany the depolarization or excitation of a cardiac muscle tissue (myocardium). The depolarization of the myocardium is usually immediately followed by a cardiac contraction. For the purpose of the present application, depolarization and contraction may be considered as simultaneous events and the terms “depolarization” and “contraction” are used herein as synonyms.
In order to monitor the heart chamber and thus to determine whether or not a natural contraction of a heart chamber has occurred a pacemaker has a sensing stage which during operation of the pacemaker is connected to an electrode placed in a respective heart chamber. A natural contraction of a heart chamber can be detected by evaluating electrical potentials sensed by such sensing electrode. In the sensed electrical signal the depolarization of an atrium muscle tissue is manifested by occurrence of a signal known as “P-wave”. Similarly, the depolarization of ventricular muscle tissue is manifested by the occurrence of a signal known as “R-wave”. A P-wave or an R-wave represents an atrial event or a ventricular event, respectively, in the further course of this application.
In a demand mode of operation, the pacemaker monitors the heart for the occurrence of P-waves and/or R-waves. If such signals are sensed within a prescribed time period or time window, which is called atrial or ventricular escape interval, respectively, then the escape interval is reset (i.e., restarted) and generation of a stimulation pulse is inhibited and no unnecessary stimulation pulse is triggered. The escape interval is measured from the last heartbeat, i.e., from the last occurrence of an intrinsic (sensed) atrial event (P-wave, A-sense, AS) if the atrium is monitored, or an intrinsic (sensed) ventricular event (R-wave, V-sense, VS) if the ventricle is monitored, or the generation of a stimulation pulse (V-pace, VP; A-pace, AP) if no respective intrinsic event has occurred. If the escape interval “times-out”, i.e., if a time period equal to the escape interval has elapsed without the sensing of a P-wave and/or R-wave (depending upon which chamber of the heart is being monitored), then a stimulation pulse is generated at the conclusion of the escape interval, and the escape interval is reset, i.e., restarted. In this way, the pacemaker provides stimulation pulses “on demand,” i.e., only as needed, when intrinsic cardiac activity does not occur within the prescribed escape interval.
Several modes of operation are available in a state of the art multi mode pacemaker. The pacing modes of a pacemaker, both single and dual or more chamber pacemakers, are classified by type according to a three letter code. In such code, the first letter identifies the chamber of the heart that is paced (i.e., that chamber where a stimulation pulse is delivered), with a “V” indicating the ventricle, an “A” indicating the atrium, and a “D” indicating both the atrium and ventricle. The second letter of the code identifies the chamber wherein cardiac activity is sensed, using the same letters, and wherein an “O” indicates no sensing occurs. The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting (“I”) response wherein a stimulation pulse is delivered to the designated chamber at the conclusion of the appropriate escape interval unless cardiac activity is sensed during the escape interval, in which case the stimulation pulse is inhibited; (2) a Trigger (“T”) response wherein a stimulation pulse is delivered to a prescribed chamber of the heart a prescribed period of time after a sensed event; or (3) a Dual (“D”) response wherein both the Inhibiting mode and Trigger mode may be evoked, e.g., with the “inhibiting” occurring in one chamber of the heart and the “triggering” in the other.
To such three letter code, a fourth letter “R” may be added to designate a rate-responsive pacemaker and/or whether the rate-responsive features of such a rate-responsive pacemaker are enabled (“O” typically being used to designate that rate-responsive operation has been disabled). A rate-responsive pacemaker is one wherein a specified parameter or combination of parameters, such as physical activity, the amount of oxygen in the blood, the temperature of the blood, etc., is sensed with an appropriate sensor and is used as a physiological indicator of what the pacing rate should be. When enabled, such rate-responsive pacemaker thus provides stimulation pulses that best meet the physiological demands of the patient.
Multiple-mode, demand-type, cardiac pacemakers shall allow a sequence of contractions of the heart's chamber which equals as far as possible a natural behavior of the healthy heart for damaged or diseased hearts that are unable to do so on their own.
In a healthy heart, initiation of the cardiac cycle normally begins with depolarization of the sinoatrial (SA) node. This specialized structure is located in the upper portion of the right atrium wall and acts as a natural “pacemaker” of the heart. In a normal cardiac cycle and in response to the initiating SA depolarization, the atrium contracts and forces the blood that has accumulated therein into the ventricle. The natural stimulus causing the atrium to contract is conducted to ventricle via the atrioventricular node (AV node) with a short, natural delay, the atrioventricular delay (AV-delay). Thus a short time after an atrial contraction (a time sufficient to allow the bulk of the blood in the atrium to flow through the one-way valve into the ventricle), the ventricle contracts, forcing the blood out of the ventricle to body tissue. A typical time interval between contraction of the atrium and contraction of the ventricle might be 180 ms; a typical time interval between contraction of the ventricle and the next contraction of the atrium might be 800 ms.
Thus, in a healthy heart providing proper AV-synchrony an atrial contraction (A) is followed a relatively short time thereafter by a ventricle contraction (V), that in turn is followed a relatively long time thereafter by the next atrial contraction and so on. Where AV synchrony exists, the heart functions very efficiently as a pump in delivering life-sustaining blood to body tissue; where AV synchrony is absent, the heart functions as an inefficient pump.
To mimic the natural behavior of a heart, a dual-chamber pacemaker, in conventional manner, defines a basic atrial escape interval (AEI) that sets the time interval for scheduling an atrial stimulation pulse. The atrial escape interval can be started by a ventricular event and end with an atrial event. A basic AV delay (AVD) or ventricular escape interval (VEI) sets the time interval or delay between an atrial event and a ventricular event. In such embodiment, AEI and AVD (or VEI) thus together define a length of a heart cycle which is reciprocal to the pacing rate at which stimulation pulses are generated and delivered to a patient's heart in the absence of sensed natural cardiac activity.
For the purpose of this application, a “ventricular event” may refer either a natural ventricular excitation (intrinsic ventricular event) which is sensed as an R-wave or a ventricular stimulation pulse (V-pulse, VP). Similarly, an atrial event shall refer to both, a P-wave or an atrial stimulation pulse (A-pulse, AP).
Since the atrial escape interval usually defines the time of delivery of a next scheduled atrial stimulation pulse, and since an atrial stimulation pulse may be timed from the latest ventricular event as well as from the latest atrial event, in some cases the atrial escape interval is an A-A interval.
One basic parameter of a heart stimulator's operation is stimulation rate. The stimulation rate is the V-V interval or the A-A interval the heart stimulator is applying. In modern heart stimulators the stimulation rate is often time variable in order to meet a hemodynamic demand of a patient that depends on the patient's physical activity. A hemodynamic sensor or activity sensor can be provided to adapt the actual stimulation rate to an actual hemodynamic demand. A heart stimulator allowing such rate adaption is called rate adaptive. Usually the actual stimulation rate is elevated compared to a base (minimum) stimulation rate. The base stimulation rate is applied whenever a patient is at rest. In order to mimic a natural circadian rhythm different base stimulation rates are provided for daytime (daytime base stimulation rate) when the patient is expected to be awake and night time when the patient is expected to sleep (nighttime stimulation rate).
Some parameters of an implantable medical device impact the lifestyle of a patient. An example is the time at which an implantable pulse generator (IPG) transitions from nighttime stimulation rate to daytime base stimulation rate. The former is generally lower than the later and thus provides less hemodynamic support. A pacemaker dependent person may thus feel less energetic if he or she wakes up before the programmed transition time.
All parameters may be programmed by the physician at follow-up using a device known as “physician programmer”. The follow-ups typically occur every three to six months. The physician may not have the time to discuss all the lifestyle impacting parameters with the patient. Even if time is allocated to this task during the follow-up, adapting the parameters to the lifestyle of the patient only two to four times a year may not provide sufficient granularity to react to changes that can potentially occur daily.